TA. 
by means of a single two-sided special cell, which is often estab- 
lished very early (See Fig.1-a), and may persist until the pro- 
thallium has reached a considerable size. This cell has two sets 
of segments cut off from its sides, by walls which are parallel to 
the latter. This regular succession of segments is usually con- 
spicuous and the limits of the individual segments are often 
recognizable for a long time (Fig. 2). Sooner or later the apical 
cell is divided by a wall at right angles to the axis of the pro- 
thallium into a marginal cell, four-sided when seen from above ; 
and an inner triangular cell (Fig. 6). The former of these next 
divides in most cases by a longitudinal wall into two equal cells, 
but in some cases observed (in Osmunda cinnamomea) the single 
four-sided cell may persist for a time as a single apical cell. 
(Figs. 4, 5). 
All of these points are readily made out from surface views 
of the prothallium, and with the exception of the last mentioned, 
have been repeatedly observed and recorded by numerous writers ; 
but the fact that in the later stages of the apical cell (or cells), 
there is a perfect regularity in the succession of segments cut off, 
seems to have entirely escaped the attention of these investiga- 
tors. Kny* it is true, figures, in the main correctly, a longitudinal 
section of the prothallium of Osmunda regalis, showing the apical 
cell and its younger segments; but makes no reference to having 
detected any regularity in the arrangement of the cells, simply 
stating that walls are formed parallel to the surface of the pro- 
thallium, thus giving rise to the thickened midrib characteristic 
of the prothallium of the Osmundaceze. 
When we compare the process of growth here briefly outlined 
with that of the thallose liverworts, we cannot but be struck with 
the remarkable resemblances—nay, identity in many cases—of the 
two. 
The liverworts have been repeatedly and carefully studied, 
especially by Leitgeb, and in his magnificent workt upon them, 
we have ample means for comparing their development with that 
of the fern-prothallia. Recently also, Knyt has published an 
* Kny—Entwickelung des Vorkeimes von Osmunda regalis. PI. 11. fig. 2. 
+ Leitgeb—Untersuchungen ueber die Lebermoose. 1874-1881. 
$ Kny—Bau und Entwickelung von Marchantia polymorpha, 1890. 
